Phase shifters are common components employed in a variety of wireless communication applications. For example, a phased-array receiver requires phase shifters to achieve desired beamforming. Particularly, a phase shifter is one of the key components for non-light of sight (NLOS) applications. Please refer to FIG. 1, which is a diagram illustrating a conventional reflection-type phase shifter. The conventional reflection-type phase shifter 100 includes a quadrature coupler 102 and a plurality of load devices 104A and 104B. As shown in FIG. 1, the quadrature coupler 102 includes an input port denoted by P1, a through port denoted by P2, a coupled port denoted by P3, and an isolated port (output port) denoted by P4. The quadrature coupler 102 is also called 90 degree hybrid coupler used for dividing an input signal S_IN into two signals 90 degrees out of phase.
The load devices 104A and 104B are implemented to serve as reflection loads, respectively. Each of the conventional load devices 104A and 104B includes a transmission line L, a bypass capacitor Cbypass, and a plurality of varactors C1. The signals respectively reflected from the load devices 104A and 104B are combined at the isolated port (output port) P4, resulting in an output signal S_OUT induced at the isolated port (output port) P4. The conventionalreflection-type phase shifter 100 can be used to provide any desired phase shift through properly tuning the capacitive values (capacitance values) of the varactors C1 implemented in the load devices 104A and 104B. In other words, the conventional phase shifter 100 employs the varactors C1 to provide phase tuning functionality for steering the signal beam angle.
However, the phase tuning curve of the phase shifter 100 is non-linear due to the varactor's non-linear C-V curve, as shown in FIG. 2. As can be seen from FIG. 2, the capacitance value C is increased/decreased non-linearly when the control voltage V applied to the varactor is increased/decreased. Specifically, when the control voltage V is within a specific range, the slope of the C-V curve is sharp, implying that a digital-to-analog converter (DAC) for setting the control voltage V of the varactor is generally required to have high resolution to properly set a desired capacitance value to the varactor. A digital compensation scheme is therefore proposed to apply pre-distortion to the incoming digital control value, thereby making the overall phase tuning curve become more linear, if the pre-distortion characteristic of the digital control value is taken into consideration.
Linearizing the phase-tuning curve of the phase shifter 100 can simplify the actual implementation of the digital compensation scheme, and save the digital-block area (e.g., a lookup table) accordingly. Besides, as the DAC conversion noise is severe when the control voltage of the varactor is within the aforementioned specific range, where the slope of the C-V curve is sharp, linearizing the C-V curve to reduce the slope can effective reduce the DAC conversion noise passed to the following signal processing stage(s).
Therefore, an innovative and efficient means to linearize the non-linear characteristic of an application device (e.g., the non-linear phase-tuning curve of the phase shifter) caused by the varactor's non-linear C-V curve is highly demanded.